1. Field of the Invention
The present invention relates to a circuit of a color TV receiver for separating a luminance (Y) signal and a chrominance (C) signal from a composite video signal and for outputting the separated signals, and in particular, to a circuit for separating the luminance and chrominance signals by use of a comb filter which can properly separate the Y and C signals even in a contour section of a raster pattern scanned on a face plate of a picture tube of the color TV receiver.
2. Description of the Prior Art
Conventionally, in the color TV signals of the NTSC system, the band containing the signals is compressed by interleaving the chrominance (C) signal in spectral spaces existing in a high-frequency region of the luminance (Y) signal. The TV receiver must separate the Y and C signals from the received color TV signal. As a circuit for separating the Y and C signals, there have been known a circuit utilizing a low-pass filter and a high-pass filter and circuit using a comb filter having a scanning line memory.
FIG. 7 is a block diagram showing a conventional circuit for separating the Y and C signals by use of a comb filter.
In general, for the TV signals, there exists a correlation between the adjacent scanning lines of the scanned raster; moreover, the phase of the chrominance signal in the NTSC system is inverted for each horizontal scanning line. The circuit shown in FIG. 7 performs the separation of the Y and C signals by use of these characteristics.
FIG. 8 is a graph illustrating signal waveforms at the respective sections of the circuit shown in FIG. 7.
Referring to FIGS. 7-8, the circuit operation will be described. For the composite video signal inputted from terminal 1, when the signal of line 1+1 is on an output line 301 of a 1H delay line 3, the signal of line 1 is on an output line 202 of a 1H delay line 2, and the signal of line 1--1 is on an input line 201 of a 1H delay line 2.
The signals on the respective lines are multiplied by -1/4, 1/2, and -1/4 in multipliers 4, 5, and 6, respectively. FIG. 8 (B) shows the resultant signals.
The minus sign for the multiplication means the phase inversion.
Next, the signals outputted from the multipliers 4-6 are added by an adder 7 and the resultant signal is fed to a band-pass filter 8 to remove the direct current (DC) component. As a result, the C signal of the video signal on the line 1 is extracted as shown in FIG. 8 (C). On the other hand, the subtractor 9 is a circuit to remove the C signal extracted from the video signal on the line 1. At an output terminal 11 of the subtractor 9, only the Y signal is attained as shown in FIG. 8 (D).
As described above, the circuit for separating the Y and C signals by use of the conventional comb filter obtains the necessary signals based on the correlation between the adjacent scanning lines on the raster of the screen. In a case where the contour section of an image is configured by lateral lines and inclined lines for which the correlation is weak in the vertical direction, the phase of the chrominance signal is not inverted between the adjacent scanning lines. In this case, therefore, the sufficient function to separate the Y and C signals cannot be developed and hence disturbance such as the cross color and dot crawl may take place.
The disturbance will be described with reference to FIGS. 9-10.
FIG. 9 is a diagram showing the contour section configured by lateral lines in which as indicated by a dotted line S, the color is changed between the lines 1 and 1+1 so as to form the contour section. Consequently, the phase of the chrominance signal should be inverted between the lines 1 and 1+1; however, the phase is not changed in this case and the complementary relationship exists between the associated colors.
As a result, through the completely same steps as those described with reference to FIGS. 7-8, the C and Y signals of the video signal on the line 1 are separated. However, the amplitude of the C signal becomes half that of FIG. 8, namely, another half thereof leaks into the Y signal, which causes so-called dot disturbance.
FIG. 10 is a diagram showing a contour section formed by inclined lines in which the phase and thus the color changes at the location indicated by a dotted line S, thereby forming the contour section. Also in this case, the C signal cannot be reproduced or separated with a high fidelity in the contour section and a portion of C signal is also mixed with the Y signal.
As described above, in the conventional Y and C signal separation circuit using the comb filter, the Y and C signals cannot be completely separated in the contour section configured by lateral lines or inclined lines, which leads to a problem that disadvantages such as the dot disturbance take place and thus the resolution of the displayed image is deteriorated.